The 3rd Generation Partnership Project (3GPP) is a collaboration agreement that brings together a number of telecommunications standards bodies. Within the 3GPP workgroups a new system concept denoted Long Term Evolution (LTE) and System Architecture Evolution (SAE) are being standardized. The architecture of the 3GPP LTE/SAE system (denoted LTE here after), which is schematically illustrated in FIG. 1, is flat compared to e.g. GSM (Global System for Mobile communications) and WCDMA (Wideband Code Division Multiple Access) based systems. FIG. 1 shows that the LTE radio base stations 100a, 100b, 100c (denoted eNodeBs, or eNBs, in 3GPP terminology) are directly connected to the core network nodes 101a, 101b MME/S-GWs (mobility management entity/serving gateway) via the S1 interfaces 102a, 102b, 102c, 102d. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs. There is no central radio network controller in the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). Instead the eNBs are connected to each other via the direct logical X2 interfaces 103a, 103b, 103c. A mobile phone being operated in such a system is an example of a user equipment denoted UE (not shown in FIG. 1).
In such systems, one example being, LTE-Advanced, multiple component carriers are aggregated in uplink and downlink, respectively. For example, to provide high data rates, the UE is configured to receive simultaneous transmissions on multiple downlink component carriers. For one UE, each component carrier is used for transmission of one transport block (two transport blocks in the presence of Multiple Input and Multiple Output, MIMO, systems). To receive such a transport block the UE must first detect that a block is incoming, more on this below. If the detection of a transport block is successful, the UE is configured to send an acknowledged message, an ACK, on the uplink and if the detection was unsuccessful, the UE is configured to send a not-acknowledged message, a NACK. Thus, with carrier aggregation, multiple ACK or NACK bits need to be transmitted from a UE in response to the transmitted transport block over different component carriers.
In the LTE standard, both the terms component carrier and cell exist. The term component carrier is related to a carrier frequency which is a term typically used when arranging measurements, and UEs then report cells on that carrier frequency. The term cell is used for many other instances, such as mobility, which refers to a change of serving cell. A cell may include both an uplink and a downlink direction of communication. For example, a UE can be assumed to have a Primary Serving Cell (PCell). In the DL, the carrier corresponding to the PCell is the DL Primary Component Carrier (PCC) while in the UL it is the UL PCC. Similarly, Secondary Serving Cell (SCell) may be configured together with the PCell. In the DL, the carrier corresponding to the SCell is the DL Secondary Component Carrier (SCC) while in the UL it is the UL SCC. Hence, carrier aggregation can equivalently be expressed as the aggregation of serving cells. In this document, we assume the notion of carrier aggregation by means of component carriers, but a person skilled in the art could equivalently use the terminology of cells instead of component carriers in relation to carrier aggregation and transmission.
As stated above, the UE needs to detect a downlink control channel before detecting the transport block. Such a downlink control channel contains the downlink assignment information needed to receive the data channel and to decode the transport block. If the UE does not correctly receive the control channel, the UE is not aware of that it is expected to receive any data channel and it does not send any ACK or NACK on the uplink. This is referred to as discontinuous transmission (DTX). The eNB knows when to expect a NACK or ACK and the eNB would have to initiate a re-transmission upon DTX detection.
Furthermore the ACK/NACK signalling in the uplink may be erroneous. For example a transmitted ACK may be received as a NACK, or a transmitted NACK may be received as an ACK. Such a NACK-to-ACK (or ACK-to-NACK) error may introduce HARQ buffer corruption due to an erroneous combination of several transmissions. An ACK-to-NACK error leads to inefficient system operation due to unnecessary retransmissions. It is therefore important to provide robust ACK/NACK signalling. To assure proper system performance, the LTE specifications list requirements on the ACK/NACK error performance.
Channel selection is one method that is capable for transmission of multiple ACK and NACK bits. The transmission is performed by Quadrature Phase-Shift-Keying, QPSK, modulated sequences and the ACK/NACK information is encoded by both the selection of the channel, in the form of sequence, and the QPSK constellation point, i.e., the modulation symbol. The channel selection refers to the selection of the sequence and several channels can be transmitted on the same frequency resource. That is, channels are obtained by Code Division Multiplexing, CDM, of sequences. Since only one sequence is selected and transmitted for one UE, channel selection preserves the single-carrier property of the signal. This ACK/NACK feedback principle was used already in LTE Rel-8 for Time Division Duplex, TDD, where ACK/NACKs from multiple downlink subframes are signalled by one transmission in a single uplink subframe. This is in the standard referred to as transmission of ACK/NACK multiplexing. For LTE-Advanced, channel selection will also be used, but in the context of conveying ACK/NACKs from multiple component carriers. This applies for UEs with maximum capability of four ACK/NACK bits and also includes the Frequency Division Duplex, FDD, case. Each transport block generally requires one ACK/NACK bit, thus four ACK/NACK bits could, e.g., correspond to a configuration of two component carriers with MIMO transmission on each carrier.
To encode the ACK/NACK/DTX information, a mapping is needed between the different states of ACK, NACK and DTX and the channels and QPSK constellation points. This can also be referred to as an ACK/NACK codebook. These tables exist in the standard for channel selection for TDD in Rel-8, but new mappings (i.e. codebooks) are needed for carrier aggregation in LTE-Advanced because of a different resource reservation, i.e., the determination of the channels will be different. Transmission of ACK/NACKs for carrier aggregation by means of channel selection requires a codebook where for each valid combination of ACK/NACK/DTX, one channel and one constellation point should be assigned. The codebook design impacts the ACK-to-NACK and NACK-to-ACK error probabilities.
Thus, an alternative manner of providing a codebook leading to an improved HARQ feedback would be useful.